Well known are the advantages of using laser sources to illuminate objects for microscopic observation compared with using conventional sources or light-emitting diodes (LEDs). More specifically, laser illumination devices are characterized by very high brightness, high image contrast, wide color gamut, miniature dimensions, and high performance efficiency. In spite of these advantages, laser illumination devices have not yet achieved widespread application primarily because of a fundamental phenomenon that leads to microscopic image degradation, i.e., observation of a floating granular pattern in front of the image plane. This pattern is known as a speckle pattern that occurs from interference of light waves having different phases and amplitudes but the same frequency. The interaction of these waves produces a resultant wave, the amplitude and intensity of which varies randomly.
The speckle formation phenomenon can be explained in more detail as follows. When the surface of an object is illuminated with coherent light, e.g., with laser light, each point of the illuminated surface acts as a secondary point light source that reflects and scatters a spherical wave. However, since the illuminated surface has its own surface microstructure, these waves will have different phases and amplitudes. More specifically, in the majority of cases, the reflecting or light-passing surfaces that constitute the objects of observation have a roughness that is comparable to the wavelength of the illumination light. It can be assumed that the main contribution to the scattering of light is made by mirror reflections on small portions of a surface. As the roughness and size of an illuminated area increases, the number of light illuminating points also increases. Propagation of such reflected (transmitted) light to the point of observation leads to interference of dephased but coherent waves at that point. As a result, the observer sees a granulated or speckled pattern. In other words, speckles comprise an interference picture of irregular wavefronts that is formed when a coherent light falls onto a heavily roughened surface.
There exist both objective and subjective speckles. Objective speckles are formed in the entire space from the source of light to the illuminated surface. The picture of objective speckles can be seen, e.g., if a high-resolution visual sensor is placed at any point of the aforementioned space on the path of illumination light. However, if we observe an object illuminated by the same light, e.g., through a microscope, we see a picture of subjective speckles. Such a picture is called subjective since its parameters depend on the optical system of the microscope. This phenomenon does not change if we increase magnification. However, the greater the aperture, the thinner the speckled structure becomes since an increase in aperture decreases the diameter of the diffraction picture created by the microscope.
Thus, formation of speckles essentially restricts the scope of application of laser illumination devices in fields such as microscopy, vision with laser illumination, optical metrology, optical coherent tomography, etc. Quantitatively, speckles are usually evaluated by speckle contrast. The speckle contrast C is usually defined as the ratio of the standard deviation α of the intensity I to the mean intensity [I] of the speckle pattern:C=σ/[I]=√{square root over ( )}([I2]−[I]2))/[I]  (1)
For a static speckle pattern, under ideal conditions (i.e., when monochromatic and polarized waves are completely free of noise) the standard deviation σ equals the mean intensity [I] and the speckle contrast is equal to unity, which is the maximum value for the contrast. Such a speckle pattern is termed “fully developed”. On the other hand, complete absence of speckles corresponds to spatially uniform intensity of illumination. In this case standard deviation σ is equal to √{square root over ( )}([I2]−[I]2)=0. Thus, from formula (1) above, it is clear that speckle contrast may change from 1 to 0.
It is understood that a laser-type illuminator that provides illumination of an object with speckle contrast equal to or close to 0 may be considered as an ideal illumination light source.
Speckle contrast is reduced by creating many independent speckle patterns that are averaged on the retina of an eye or in a visual sensor. Speckle contrast can be reduced by changing illumination angle or by using different polarization states, laser sources with close but still different wavelengths, rotating diffusers, or moving or vibrating membranes that are placed on the optical path of the illuminating light. Many practical methods based on the aforementioned ideas for speckle contrast reduction are known and disclosed in patents, published patent applications, and technical literature. However, practically all of these methods are based on averaging independent speckle patterns created by light that is the same but that propagates along different optical passes.